Electrical machine

ABSTRACT

An electrical machine comprising a stator and a member arranged to move relative to said stator. The stator comprises a first stator structure incorporating a first plurality of windings disposed along the direction of relative motion and displaced from the movable member in a direction perpendicular to the direction of motion, and a second stator structure incorporating a second plurality of windings displaced in a second direction from the movable member, so that the movable member lies between the two stator structures. The movable member is arranged to electrically couple with the windings. The first plurality of windings includes a single phase winding connected to a first single-phase ac supply, and the second plurality of windings includes a single phase winding of the same pole pitch being displaced in the direction of relative motion from first plurality of windings and connected to a second single-phase ac supply whose phase is displaced from that of the first supply.

RELATED APPLICATIONS

The present invention is a U.S. National Stage under 35 USC 371 patent application, claiming priority to Serial No. PCT/EP2013/072558, filed on 29 Oct. 2013; which claims priority from European Application No. 12190733.1, filed 31 Oct. 2012, the entirety of both of which are incorporated herein by reference.

FIELD

The present invention relates to an electrical machine.

BACKGROUND

In many types of electrical machine, electric currents in a stator create a travelling magnetic field that interacts with a set of electric currents, a set of permanent magnets or a set of ferromagnetic features on the moving part of the machine, i.e. a rotor in the case of a rotary machine, or a translator in the case of a linear machine. The usual method of creating the travelling field is to use three component windings housed together in a uniform array of slots in a laminated iron stator and fed with three alternating currents from a three-phase electrical power supply. Each component winding has a cyclic distribution of coils and the three component windings are placed with their magnetic axes spaced at intervals of ⅓ of a wavelength along the stator. The alternating currents fed to the windings have relative phase differences of 120 degrees. As a result, the combined magnetic flux approximates a sinewave of constant amplitude travelling at a speed equal to one wavelength of the cyclic distribution during one cycle of the alternating current. Other polyphase windings are possible but are seldom used because the prevalent form of power supply is the three-phase type.

The most common type of electrical drive, the three-phase induction motor of the type used in many industrial applications, employs this method to produce a rotating field. Referring to FIG. 1(a), a rotor 12 carries a set of conducting bars (not shown) in which electric currents are induced and which interact with the rotating field to produce torque on an output shaft.

In domestic and light industrial applications, where a three-phase electrical supply is not available, an alternative arrangement is used whereby two component windings are supplied with alternating currents differing in phase. The first current is provided directly by an available single-phase supply and the second is obtained from the same supply usually via a capacitor that introduces a phase shift. The phase shift is of the correct degree only under one load condition so under most conditions the operation of such motors is not ideal. Such machines are referred to as single-phase motors because they operate from a single-phase supply but the windings are more accurately described as two-phase windings. If a balanced two-phase supply providing two currents displaced in phase by 90 degrees were available, electrical machines of this type with component windings displaced by ¼ of the wavelength would create a rotating field of constant amplitude and could be just as effective as standard three-phase machines.

In recent years, linear electric motors have attracted interest for several applications including guided ground transport and for electromagnetic launch systems on aircraft carriers. Also linear generators have been used in certain wave-power devices relying on reciprocating movement. A linear electrical machine can be considered as a standard rotary machine that has been cut and un-rolled, as shown in FIG. 1(a) . . . (c). When this is done, the magnetic force of attraction indicated by the force lines 16 between stator 10 and translator 14 is no longer balanced by equal and opposite forces as in the case of the stator 10 and rotor 12.

A common approach to overcome this problem is to use two stators 10′, 10″ placed on opposite sides of a single translator 14 as shown in FIG. 2. In most cases magnetic flux 18 passes from the first stator 10′, through the translator 14, through the second stator 10″, and through the translator for a second time to complete its circuit.

The two-stator linear machine could be rolled up around the original axis (shown as A in FIG. 1(b)) to form a rotary machine with two co-axial stators enclosing a hollow cylindrical rotor and with magnetic flux passing radially from the inner stator, through the inner gap, the cylindrical rotor and the outer gap to the outer stator. Machines of this configuration have applications in servo control systems where a drive of very low inertia is required.

Alternatively, the linear machine may be rolled up around an axis orthogonal to the original axis (shown as B in FIG. 2) to create a rotary machine where the flux passes axially across two planar airgaps rather than radially through cylindrical airgaps. Axial-flux rotating machines of this type have been used particularly as permanent-magnet generators for renewable energy applications, notably small wind turbines. In such cases each of the two stators normally carries a three-phase winding comprising three component windings as described above.

It is an object of the present invention to provide an improved electrical machine.

SUMMARY

According to the present invention there is provided an electrical machine as claimed in claim 1.

In embodiments of the invention, a two-stator machine has a two-phase winding in which one component winding is housed in each stator and the two component windings are aligned with their magnetic axes displaced by approximately 90 electrical degrees.

The displacement between the magnetic axes of the two windings is ideally 90° and the windings are ideally supplied with alternating currents that differ in phase by 90°. The invention applies to motors or generators in linear or either radial- or axial-flux rotary form. Each stator carries a single winding and this removes the restrictions caused by each winding having to share a common array of slots. As a result:

-   1. The winding distribution can be arranged to approximate closely     to a sinusoidal pattern because the slots no longer need to be     uniformly distributed and can be distributed according to the     requirements of just the one winding housed in them. -   2. The winding layout is simplified because the winding does not     need to overlap other windings and so it may be a straightforward     set of planar coils in the case of a linear machine or an axial-flux     rotary machine -   3. The insulation is simpler because the winding needs only to be     isolated from ground and not from other windings. -   4. As a consequence of 2 and 3 it becomes practicable to form each     winding from a continuous length of insulated cable. For undersea     applications, this is especially beneficial as it simplifies the     waterproofing the stator electrics as all winding connections can be     readily made within a single sealed unit.

The movable member, whether a rotor or translator, may comprise a conducting body so that the machine operates as an induction motor or generator. Such an induction generator is tolerant to hypocycloidal motion produced by movement of a shaftless rotor within a turbine of the type shown in FIGS. 3-5. The rotor or translator may also include ferromagnetic parts to assist the passage of magnetic flux between the two stator structures.

Alternatively, the rotor or translator may comprise an array of permanent magnets or electromagnets so that the machine operates as a synchronous machine. With appropriate control of the electrical feed, such a machine would form a so-called brushless-dc machine.

A further alternative is to use a rotor or translator that includes an array of ferromagnetic features that interact with the magnetic field of the stator to create torque in the case of a rotary machine or thrust in the case of a linear machine. With appropriate control of the electrical feed, such a machine could form a synchronous reluctance machine, a switched-reluctance machine or a stepper motor.

The two windings may be fed from a two-phase electrical source or, in the case of a generator, they may feed into a two-phase load. A two-phase source may be provided by:

-   1. a special-purpose two-phase alternator; -   2. a power-electronic converter with two-phase output, such as may     be arranged using two H-bridges using IGBTs and connected to a     common dc link; -   3. a three-phase source with a set of transformers connected to     convert between three-phase and two-phase systems, such as a Scott-T     connected transformer; or -   4. a single-phase source with a phase-shifting impedance as used in     a single-phase motor.

A two-phase load for a generator may employ one of the above arrangements operating with reversed power flow or it may comprise simply resistive load banks. In the case of a long linear machine or a large-diameter rotary machine, it is preferred to divide the stators into a number of manageable sections. For a rotary machine, the sections may be a set of arcs disposed around the machine or alternatively if a large number of sections is used, then in the case of an axial-flux machine it may be convenient and more cost effective to make the sections straight and disposed tangentially. If the sections are spaced apart, then the emfs in their windings differ in phase. If the distance between sections is ⅓ of a wavelength then the phase difference is 120 degrees and if the stator has been divided by a multiple of three, then a three-phase supply can be used to supply the sections of one stator. If the distance is ⅙ of a wavelength then a three-phase source may be used with the windings of alternate blocks reversed. In either case, a second three-phase supply with its three outputs in phase quadrature with those of the first three phase supply can be used to supply the sections of the second stator. If the windings of the sections of the first stator are connected in star, then the required second three-phase supply can be provided by connecting the windings of the second stator in delta and to the same three-phase source. In this way a standard three-phase source can be used. It is noted that the voltage applied to the windings connected in delta is higher than the voltage applied to the windings connected in star and the current is less. The windings of the two stators are therefore preferably arranged to operate at different voltage and current by specifying proportionately different numbers of turns of conductor of different cross section.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIGS. 1(a) to (c) show schematically the components of a rotary and a linear electrical machine;

FIG. 2 shows a two-stator linear machine;

FIG. 3 shows in elevation a housing for an open centered hydro-turbine including an electrical generator according to an embodiment of the present invention;

FIG. 4 shows a perspective view of the housing of FIG. 3;

FIG. 5 is a cross-sectional view through the line V-V and the plane V-V indicated in FIGS. 3 and 4;

FIG. 6 shows an exemplary layout of windings on a stator for the turbine of FIGS. 3-5;

FIGS. 7(a) and (b) show in detail the layout of windings for a pole of the stator of FIG. 6;

FIG. 8 shows a cross-section of an exemplary laminate for accommodating the windings of FIGS. 6 and 7;

FIG. 9 shows a cross-section of an alternative laminate for accommodating windings for a stator;

FIG. 10 shows an electrical connection for the windings of the two stators arranged to operate with a standard three-phase electrical system;

FIG. 11 shows a two-bridge converter for use with the system of FIG. 10; and

FIG. 12 shows a converter providing a two-phase output for use in alternative implementations of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One particular application for the present invention is an axial-flux, low-speed, direct-coupled, rotary generator for use in an open-centre, shaftless, tidal-current turbine. Referring now to FIGS. 3 and 4, there is shown such a turbine 30 including an electrical motor/generator according to an embodiment of the present invention. The turbine comprises a runner, comprising a set of blades 32 that are fixed between an inner ring 34 and an outer ring 36, housed in a shroud 38. Typically, the turbine housing also includes mounts which allow the turbine to be secured to a subsea structure such as a gravity base—although this detail is not shown in the present case.

Turning now to FIG. 5 which shows a portion of the turbine in cross-section. The outer ring 36 runs within a channel defined by a front annular support 40, an outer cylindrical support 42 and a rear annular support 44—front and rear being defined relative to the movement of fluid through the turbine indicated by the arrow F. Nonetheless, it will be appreciated that the turbine can be bidirectional and so responsive to fluid flow in either direction. Each support 40, 42, 44 is separated from the ring 36 by a respective bearing 40′, 42′, 44′. In the embodiment, none of these components need be magnetic and can be designed purely from a mechanical transmission efficiency point of view.

In this embodiment, a motor/generator assembly is disposed axially away from the turbine assembly. The motor/generator assembly again comprises a front annular support 46, a cylindrical outer support 48 and a rear annular support 50. The front annular support 46 can be fixed to the rear annular support 44 or indeed these can be produced as an integral component. A first circular stator comprises a set of coils 52 wound around a circular laminate 53 as described in more detail below and the laminate 53 is fixed to the front support 46. A second circular stator comprises a set of coils 54 wound around a circular laminate 55 again as described in more detail below and the laminate 55 is fixed to the rear support 50. An annular rotor 56 is disposed between the two stators 52, 54 and this motor/generator rotor 56 is directly coupled to the outer ring 36 of the runner via a transmission ring 60. Thus, when the motor/generator 52, 54, 56 is driven to start the turbine, rotation is transmitted to the runner via the ring 60, and once running, the runner drives the motor/generator via the ring 60. It will nonetheless be appreciated that in the right tidal conditions, tidal flow could be used to start the turbine without the need to kick-start the turbine electrically. However, the possibility of kick-starting the turbine means that it can operate across a greater range of conditions including low flow conditions than if this option were not available.

FIG. 6 shows the construction of the stators 52, 54 of FIG. 5 in more detail. In some applications, the stator could have a mean diameter of 9 m and in this case a pole number of 48, i.e. the winding 53, 55 on each stator 52, 54 is a single-phase 48-pole arrangement. While it will be appreciated that the second stator 54 is identical to and facing the first 52, in a 48 pole arrangement it is circumferentially displaced from the first by an angle of 3.75° which is half of a pole pitch.

In one embodiment, the winding of each stator is formed from 12 lengths of insulated cable laid into 864 slots. FIG. 7a illustrates the first stage 60(1) of the cable layout for one pole 60, where the number of slots per pole has been reduced from 18 to 10 in order to simplify the illustration. Cable laying progresses in a helical manner from the outside to the inside to form a single-layer, 9-turn, planar coil. Cable laying then proceeds as shown dotted in FIG. 7(a) and a second layer 60(2) is formed in the same slots in the pattern shown in FIG. 7(b), which finishes at the outside. On completion of the second layer, cable laying may proceed to form two further layers in the same slots. Alternatively, cable laying may proceed to form two layers on an adjacent pole. In the present example, each of the 12 lengths of cable is used to form a total of 16 pairs of layers and these may be arranged as 2 layers on each of 16 poles, 4 layers on each of 8 poles or 8 layers on each of 4 poles. It will be seen that the two ends of each length of cable are situated on the outside of the windings and so it is convenient to provide sufficient spare length to reach an electrical equipment enclosure (not shown) where the cable ends may be passed through watertight glands. This arrangement is favourable for subsea use as it avoids any cable joints or splices that are expensive to form and are vulnerable points where electrical faults could occur. The 12 lengths of cable may be connected in parallel to a common single-phase ac source or they may be isolated and connected to separate independent single-phase sources so that in the event of a cable failure or failure of a source, the system can continue to operate.

Each of the stator cores 53, 55 is ideally a continuous laminated ring that could be formed from a continuous spiral of electrical steel sheet. However a continuous ring of the required dimensions could be unwieldy and in some applications, it is preferred to divide the core into a number of arcs or short straight lengths that assemble to form a polygon. In the present example, the preferred arrangement has 12 arcs or straight lengths and the cable winding is formed as four poles of eight layers on each. The winding may therefore be formed before the core sections are assembled to form the complete stator ring (or polygon).

In one embodiment, slots 80 are distributed as shown in FIG. 8 with positions selected to give progressively higher concentrations of turns further from the pole axes, giving a good approximation to a sinusoidal current distribution. FIG. 8 shows sections from the two stators facing each other and displaced by a half pole pitch.

In alternative arrangements, the slots can be arranged in a uniform array of slots but with each slot containing different numbers of turns. This can be achieved by omitting some of the inner loops of cable around each pole for the first few layers. In this case, the slot depth may differ according to the number of turns allocated or the slots may be all of the same depth and those containing fewer turns may contain inert packing. The illustration in FIG. 9 shows a lamination with deep slots 90 furthest from the pole axis which could contain 8 layers of cable. The first two layers of the winding for each pole would have 2 loops. Closer to the pole axis, shallower slots allow the next two layers to have 3 loops and the last four layers contain 4 loops.

A combination of the approach shown in FIGS. 8 and 9 for achieving a sinusoidal distribution could also be adopted. The benefit of achieving a close approximation to a sine wave is the reduction in losses associated with space harmonic components of the current distribution. Harmonic components give rise to flux components that rotate at speeds different from the speed of the fundamental and induce unwanted currents within the rotor leading to additional losses.

FIG. 10 is an electrical diagram showing an exemplary electrical system for the generator. In one implementation, three windings a, b and c are connected in star to the source/grid 100 and are housed with one stator; whereas a second set of windings A, B and C are connected in delta to the source/grid and housed with the other stator. Each stator structure is split into sections with the sections arranged so that the travelling magnetic field within the machine induces single phase alternating emf in the sections displaced in phase by 120 electrical degrees. Thus, each winding a, b, c, A, B, C is connected to a respective section of each stator. The delta-connected windings A, B, C should have approximately √3 times as many turns as those in star connection because they operate with the same magnetic flux but at √3 times higher voltage. However they carry only 1/√3 as much current to deliver the same power and so their windings may be of correspondingly smaller cross section.

Exemplary power conversion circuitry 100 comprising a voltage-source inverter is described in PCT/EP2012/065701. FIG. 11 in particular shows a two-bridge converter where both a first stage converter 110 and a second stage converter 120 comprise a 3-phase voltage source inverter connected via a DC link 140. The phase ports of the first stage 110 are connected to the source/grid, whereas the phase ports P1, P2, P3 of the second stage 120 are connected to the stator blocks as shown in FIG. 10.

Referring now to FIG. 12, in other implementations, each stator's single phase winding can be connected to a power converter 110, 140, 125 comprising a respective H bridge that converts a dc source 140 to a single-phase ac source, phase A and phase B, the dc ports of the two H bridges 125 being connected to a common dc source 140 and their ac ports being arranged to provide voltages phase A and phase B, that differ in phase.

The rotor 56 is an annular sheet or plate of electrical conductor that rotates in the gap between the two stators as illustrated in FIG. 5. The annulus may be fabricated from several arcs or trapezoidal sheets. It is best to avoid using a number of sheets equal to the number of stator sections so that any torque pulsations created by the joints tend to mutually cancel. 

The invention claimed is:
 1. An electrical machine comprising a stator and a member arranged to move relative to said stator in a direction of relative motion, the stator comprising a first stator structure incorporating a first plurality of windings disposed along the direction of relative motion and displaced from the movable member in a direction perpendicular to the direction of relative motion, and a second stator structure incorporating a second plurality of windings displaced in a second direction from the movable member, so that the movable member lies between the two stator structures, the movable member being arranged to electrically couple with the windings, wherein the first plurality of windings includes a single phase winding having a pole pitch, the second plurality of windings includes a single phase winding of the same pole pitch being displaced in said direction of relative motion from first plurality of windings, wherein the displacement of the second plurality of windings with respect to the first is half of one pole pitch in said direction of relative motion, and wherein each stator structure comprises slotted laminations having a respective group of slots for each pole of the respective single phase winding, and wherein the groups of said first stator structure are displaced in the direction of relative motion from the groups of said second stator structure.
 2. The electrical machine of claim 1 wherein the plurality of windings of each stator structure is split into sections with gaps between the sections arranged so that the travelling magnetic field within the machine induces single phase alternating emf in the sections displaced in phase by 120 electrical degrees, the sections of winding being connected to one of: a three-phase ac-dc power converter or voltage source.
 3. The electrical machine of claim 2 wherein the windings of the first stator structure have more turns than those of the second stator structure by a factor of approximately square root of 3, the windings of the first stator structure being connected in delta to a three-phase power converter or voltage source and the windings of the second stator structure being connected in star to the same three-phase converter or voltage source.
 4. The electrical machine of claim 1, wherein said machine comprises a rotary machine in which the stator and the movable member are curved about an axis of rotation, the movable member comprising a rotor.
 5. The electrical machine of claim 4, wherein the stator structures are displaced axially from the rotor and the magnetic field in each region between the rotor and a stator structure passes in a substantially axial direction.
 6. The electrical machine of claim 5 wherein the stator structures are formed from sets of arc segments arranged to form at least a partially circular array about the axis of rotation.
 7. The electrical machine of claim 5 wherein the stator structures are formed from sets of substantially rectangular blocks arranged to locate tangentially to form at least a partially circular array about the axis of rotation.
 8. The electrical machine of claim 5 wherein the plurality of windings of each stator structure is split into sections with gaps between the sections arranged so that the travelling magnetic field within the machine induces single phase alternating emf in the sections displaced in phase by 120 electrical degrees, the sections of winding being connected to one of: a three-phase ac-dc power converter or voltage source.
 9. The electrical machine of claim 4, wherein the stator structures are formed from sets of arc segments arranged to form at least a partially circular array about the axis of rotation.
 10. The electrical machine of claim 4, wherein the machine comprises one of a permanent magnet synchronous machine or as a brushless-dc machine wherein the rotor comprises a cylindrical array of permanent magnets.
 11. The electrical machine of claim 1, wherein the machine comprises an induction machine in which said movable member comprises a sheet of electrically conductive material.
 12. The electrical machine of claim 1, wherein the machine comprises one of a permanent magnet synchronous machine or a brushless-dc machine in which said movable member comprises an array of permanent magnets.
 13. The electrical machine of claim 1 wherein each stator structure comprises slotted laminations, housing groups of concentric coils of electrical conductor.
 14. The electrical machine of claim 13, wherein each group of coils within each single phase winding is formed by a continuous insulated cable.
 15. The electrical machine of claim 1 wherein the respective slots of each of said first and second stator structure are non-uniformly spaced apart with the spacing between successive slots being progressively increased and then progressively decreased to create an approximately sinusoidal distribution of said slots.
 16. The electrical machine of claim 1, wherein the spacing between adjacent slots of each group of slots becomes progressively smaller in a direction towards a centre of the respective group.
 17. The electrical machine of claim 1, wherein the respective slots of each of said first and second stator structures are uniformly spaced apart with successive slots accommodating a progressively higher number of turns of the respective first or second winding and then progressively lower number of turns of the respective first or second winding to create an approximately sinusoidal distribution of turns in said slots. 